This invention relates to a light illuminating method, a surface examining method, and apparatuses for performing these methods.
The applicant has already developed and filed an improved surface examining method and an improved apparatus for performing the method, as follows (U.S. Pat. No. 5,737,074).
The surface examining apparatus 10 comprises a light illuminating device 11 and an observation device 12, as shown in FIG. 12. The light illuminating device 11 comprises a light source 11a, an aperture stop 11b, a half mirror and a lens 11d for illumination. The light beam emitted out of the light source 11a is changed to a substantial point light source by the aperture stop 11b. The light beam from the point light source is reflected by the half mirror and is collimated through the lens 11d for illumination, to illuminate an object 13 to be measured. The observation device 12 comprises a lens system 12a for observation (the lens 11d for illumination also constitutes a part of the lens system 12a for observation), and an aperture stop 12b which is located at the position of a stop of the lens system 12a for observation. The light beam reflected by the object 13 to be measured forms an image on an image pickup part 14 through the observation lens system 12a and the aperture stop 12b. The observation lens system 12a and the aperture stop 12b constitutes an object-side telecentric optical system. In the specification, the meaning of the term xe2x80x9cpoint light sourcexe2x80x9d includes not only a light source which is considered to be a perfect point but also one having a size to a small degree.
In the surface examining apparatus 10, if it is assumed that the light source is considered to be a perfect point one (i.e., the illumination angular aperture is approximately 0xc2x0) and the object-side angular aperture of the aperture stop 12b is also approximately 0xc2x0, when observing the object 13 to be measured having an approximate flat surface without inclining, the image of the flat surface of the object 13 to be measured comes to have a luminance of 100% and the image of the slope of an irregularity of the object 13 comes to have a luminance of 0%.
On the other hand, if it is assumed that the light source is considered to be a substantial point one and has a predetermined illumination angular aperture and the object-side angular aperture of the aperture stop 12b is also approximately 0xc2x0, the image of the slope of the irregularity of the object 13 comes to have a luminance which depends on the angle of the slope.
When observing the object 13 to be measured by inclining itself, it is possible to obtain the image of the slope of the irregularity of the object 13 having a luminance which depends on the angle of the slope by optionally changing the luminance of the flat surface of the object 13, for example, to 0% or 50%.
Such an observation for the surface condition of the object 13 to be measured enables measurement of the presence or absence of irregularity and the condition of the irregularity on the basis of the luminance of the observed image.
However, as a result of observation for the surface condition of the object 13 to be measured, by using the above-described surface examining apparatus 10, the existence of white or black portions (missing portion) from which portions of image were missed was recognized in a relatively wide range. Existence of such missing portions makes hard to examine the entirety of the surface of the object 13 to be measured, accurately.
The present invention was developed in view of the above-described problems. An object of the present invention is to provide a light illuminating method which is suited for, e.g., observation for the surface condition of the object to be measured and the like, to provide a surface examining method using the light illuminating method, and to provide apparatuses for performing these methods.
Generally, a lens has a spherical aberration. In practice, because of the spherical aberration, as shown in FIG. 1, after the light beams entered a lens 1 in parallel to the optical axis thereof and passed through the lens 1, the light beams do not converge at a point. That is, the image formation points on the optical axis are shifted according to the heights of light beams entering the lens 1. This is the longitudinal aberration caused by a spherical aberration. A longitudinal aberration view in the case is shown in the right portion of the Figure.
When using a lens which has a longitudinal aberration for an observation device, generally, a plurality of lenses are combined so as to have an aberration curve of the so-called full correction, as shown in FIG. 2, and an image pickup surface is set at a position of the optimal image point in the inside of the group of image formation points on the optical axis.
In a light illuminating device, although the lens therefor has no aberration curve of full correction, generally, a point light source is set at a position of the optimal image point, like the case of an observation device, instead of an image pickup surface.
FIG. 3 shows an example of such a light illuminating device. The right side in this Figure is a longitudinal aberration view in the light source side. The lens (lens for illumination) 2 shown in this figure has a characteristic with respect to a longitudinal aberration caused by a spherical aberration in the light source side, that the amount of shift from the paraxial image surface gradually decreases with an increase in height of light incidence into the lens 2. In the light illuminating device, the point light source 3 is set at a position in the inside of the group of image formation points on the optical axis.
In the light illuminating device, by tracing back along the light beams after passing through the lens 2, from the side of the point light source 3, to study the change of inclination angle of the light beams with respect to the optical axis of the lens 2 in a direction perpendicular to the optical axis, it is understood that some points of inflection in the change of inclination angle of the lights, that is, points at which a plus sign or a minus sign of inclination of the lights is changed to the other, are created. When there is such a point of inflection in the change of inclination angle of the lights, the change of inclination angles of the lights reflected by an illuminated body (one having an approximately flat surface) comes also to have a point of inflection. In the case, the number of points of inflection in the change of inclination angle is the same as that of points of inflection in the change of reflection angle in a particular direction of the illuminated body.
When there are some points of inflection in the change of reflection angle of light beams in a particular direction of the illuminated body, an observation from a certain direction comes to give a noticeable unevenness of illumination. Therefore, existence of such a point of inflection is not preferable for a light illuminating device to which a uniform illumination on a surface to be illuminated is required. For a surface examining apparatus, unevenness of illumination on the surface to be illuminated leads to an inaccurate surface examination. Particularly, in a surface examining apparatus to observe the reflected light from an object to be measured through an aperture stop or an aperture, a phenomenon of partially missed image is often caused.
In such cases, when the number of points of inflection in the change of reflection angle in a particular direction of the illuminated body is one, it is possible to reduce unevenness of illumination or to optically cut the part of the image having unevenness of illumination, by changing the position of aperture in the light illuminating optical system or the observing optical system, or the like. However, when the number of points of inflection in the change of reflection angle in a particular direction of the illuminated body is two or more, it is difficult to reduce unevenness of illumination or to optically cut the part of the image having unevenness of illumination, by changing the position of aperture, or the like.
After consideration of these points, in order to make the change of reflection angle in a particular direction of the illuminated object to be measured, have only a point of inflection, the inventors have used a lens having a characteristics with respect to a longitudinal aberration caused by a spherical aberration in the light source side of the lens for illumination, that the amount of shift from the paraxial image surface to the image formation point gradually increases or decreases, with an increase in height of light incidence into the lens for illumination; and have set the point light source at a position in the outside of the group of image formation points on the optical axis. As a result of observation of the reflected light beams from the illuminated object to be measured, through an aperture stop or an aperture, it has been ascertained that no phenomenon of partially missed image is caused.
The present invention has been developed in view of the above-described knowledge. In accordance with an aspect of the present invention, the surface illuminating apparatus for illuminating with a light beam from a light source through a lens member for illumination, is characterized in that the lens member for illumination has a characteristics with respect to a longitudinal aberration caused by a spherical aberration in the light source side of the lens for illumination, that an amount of shift from a paraxial image surface to image formation points gradually increase with an increase in height of light incidence into the lens; and that the light source is set at a position in an outside of a group of the image formation points.
In accordance with another aspect of the present invention, the surface illuminating apparatus for illuminating with a light beam from a light source through a lens member for illumination, is characterized in that the lens member for illumination has a characteristics with respect to a longitudinal aberration caused by a spherical aberration in the light source side of the lens for illumination, that an amount of shift from a paraxial image surface to image formation points gradually decrease with an increase in height of light incidence into the lens; and that the light source is set at a position in an outside of a group of the image formation points.
The above-described phenomenon of partially missed image is also caused in the case of the point light source disposed out of the optical axis of the lens member for illumination or in the case of the light source being a surface illuminant (which corresponds to the case of point light sources disposed on and out of the optical axis). In the case, a comatic aberration introduces a problem. When the influence of the comatic aberration is not negligible, it is preferable that the lens member for illumination has a spherical aberration and a comatic aberration which have approximately the same shape as each other. The reason for this is that the spherical and comatic aberrations having approximately the same shape as each other enables easy determination of the optimal position of light source according to the spherical aberration view.
Preferably, a light beam from the light source is collimated through the lens member for illumination.
In accordance with another aspect of the present invention, the surface examining apparatus comprises the above-described surface illuminating apparatus and an observation device; wherein examination for a surface condition of an object to be measured is carried out by illuminating the surface of the object by the surface illuminating apparatus and by observing a reflected light beam from the object by the observation device.
The observation device preferably comprises an object-side telecentric optical system or an image-object-side telecentric optical system.
In accordance with another aspect of the present invention, the surface illuminating method for illuminating with a light beam from a light source through a lens member for illumination, comprises the steps of: preparing the lens member for illumination which has a characteristics with respect to a longitudinal aberration caused by a spherical aberration in the light source side of the lens for illumination, that an amount of shift from a paraxial image surface to image formation points gradually increase with an increase in height of light incidence into the lens; and setting the light source at a position in an outside of a group of the image formation points. The surface illuminating method for illuminating an object with a light beam from a light source through a lens member for illumination, may also comprises the steps of: preparing the lens member for illumination which has a characteristics with respect to a longitudinal aberration caused by a spherical aberration in the light source side of the lens for illumination, that an amount of shift from a paraxial image surface to image formation points gradually decrease with an increase in height of light incidence into the lens; and setting the light source at a position in an outside of a group of the image formation points.
In accordance with another aspect of the present invention, the surface examining method for examining a surface condition of an object to be measured, comprises the steps of; illuminating the surface of the object with a light beam by the method as described above, and observing a light beam reflected from the object.
According to the light illuminating method, the surface examining method using the light illuminating method, and the light illuminating device and the surface examining apparatus, for performing these methods, it is possible to make the created inflection point at which a plus sign or a minus sign of inclination of the light beams is changed to the other, only one, when tracing back along the light beams after passing through the lens for illumination, to study the change of inclination angle of the light beams with respect to the optical axis of the lens in a direction perpendicular to the optical axis. As the result, the method or apparatus according to the present invention enables reduction of unevenness of illumination to provide an ideal illumination, in comparison with the case having two or more inflection points. Use of the invention as a surface examining apparatus further enables reduction of phenomenon of partially missed image. In particular, it is possible to eliminate unevenness of illumination or to cut the part of the image having unevenness of illumination, by changing the position of aperture in the light illuminating optical system or the observing optical system.